Industrial Platinum Metals Chemistry Towards the Year 2000

Article Synopsis

The First Anglo-Dutch Symposium, sponsored by the Royal Society of Chemistry and including the Ludwig Mond Lecture, was hosted on the 18th September 1996 by the University of Sheffield. This symposium covered a wide range of platinum group metals chemistry, illustrating in particular its continuing importance in catalysis and describing some future industrial uses, thus demonstrating the strength of the work being undertaken in both The Netherlands and the U.K.

The Ludwig Mond lectureship was established by ICI to award excellence in the field of inorganic chemistry. The present recipient, Professor Peter M. Maitlis of Sheffield University, gave this year’s lecture entitled “New Explorations in Metal Catalysed Reactions” at the First Anglo-Dutch Symposium to an audience in Sheffield of over 150 academics and industrialists.

The Ludwig Mond Lecture

Ludwig Mond was born in Germany, in 1839. After studying chemistry he came to England at the age of 23, and in 1873 with John Brunner founded the firm Brunner, Mond and Company to make sodium carbonate on a large scale using the Solvay process, which Mond improved. By the early 1880s he and Langer had discovered a process for the purification of nickel, whereby nickel was heated with carbon monoxide to form nickel carbonyl, Ni(CO)4 which then decomposed at 180°C to give pure nickel, free from carbon. In 1900 he founded the Mond Nickel company, a forerunner of ICI.

Professor Maitlis discussed three areas of his current research: methanol carbonylation, hydro-desulfurisation and Fischer-Tropsch reactions.

Methanol Carbonylation

The work of the University of Sheffield and BP Chemicals on the rhodium and iridium catalysed carbonylation of methanol to acetic acid was described. The reaction steps for both catalytic cycles are similar, but there is a difference in the reaction rates for the steps between the metals.

For rhodium, the rate determining step of the reaction is the slow oxidative addition of methyl iodide (made from HI, the co-catalyst and methanol) to the catalytically active species [Rh(CO)2(I)2]−. The previously uncharacterised reaction intermediate, [MeRh(CO)2(I)3]−, where Me is methyl, has been detected and characterised spectroscopically for the first time. The methyl rapidly migrates to form the acyl species, [(MeCO)Rh(CO)(I)3]−, which then gains a CO molecule before reductive elimination of acyl iodide occurs and [Rh(CO)2(I)2]− is regenerated. The acyl iodide is then hydrolysed to acetic acid, regenerating HI and completing the catalytic cycle.

In contrast, the oxidative addition of methyl iodide to [Ir(CO)2(I)2]− is fast, but the methyl migration is slow. However, this step, which requires temperatures over 80°C in chlorobenzene, can be dramatically accelerated by a factor of ten thousand when methanol is added. This may be due to activation of an Ir-I bond.

Hydrodesulfurisation

The hydrodesulfurisation of crude oil to remove sulfur is a massive global business. Usually a molybdenum/cobalt, Mo/Co, catalyst is used to remove aliphatic sulfur compounds, such as thiols, thioethers and dithioethers. However, the Mo/Co system has difficulty removing the sulfur contained in aromatics, whereas platinum compounds could potentially be very active here. A zero valent platinum complex, for example, can insert into a sulfuraromatic carbon bond and react further, as shown in Figure 1. The Sheffield group are aiming to make this process catalytic.

Fig. 1

Fischer-Tropsch Reaction

The Sheffield research into the Fischer-Tropsch reaction, where syngas from coal is converted into other chemical feedstocks, was discussed:

A new catalytic cycle, involving alkenyl intermediates was described by Professor Maitlis. When 13C2 labelled molecules are used as probes, it can be seen that alkenyl/vinyl species are directly involved in polymerisation on catalytic rhodium and ruthenium surfaces, see Figure 2.

Fig. 2

Scheme of the alkenyl mechanism for the Fischer-Tropsch reaction shows that initiation occurs when a surface methyne and methylene combine to give a surface vinyl. This reacts with another surface methylene to form a surface allyl. The allyl isomerises to the alkenyl, which reacts with more methylene thus increasing the chain length

Industrial Applications of Catalysis

The application of the Sheffield work to the new BP Cativa process, was discussed by M. J. Howard of BP Chemicals, Hull. In this process iridium – with advantages over the existing rhodium process – is used to catalyse the carbonylation of methanol. The iridium system has higher catalyst solubility and stability, and can tolerate a wide range of process compositions and allows higher rates of reaction than rhodium. The Cativa process can be fitted into existing plant, thus increasing capacity by over 30 per cent by “debottlenecking” – speeding up the slowest step of the process. The Cativa process is running successfully at a plant in Texas.

A new palladium catalysed polyketone synthesis was discussed by Professor E. Drent of Shell International Chemicals Ltd., Amsterdam. The catalyst is made from palladium acetate and the reaction proceeds as follows:

Professor Drent demonstrated that the reaction is much faster if n = 3 than if n = 2. The palladium complex is forced into a cis geometry by the bidentate phosphorus ligand, thus restricting its activity. When the palladium complex (PPh3)2Pd(OAc)2 is used cis/trans isomerisation can occur resulting in rapid product elimination and consequently there is no opportunity to build up a polymer. These polymers are attracting interest as a new type of high performance plastic.

The effectiveness of studying catalytic steps by model systems was illustrated by Professor K. Vrieze of the University of Amsterdam. He showed how a polyketone oligomer is built up by inserting carbon monoxide into a palladium-methyl bond. The alkene co-ordinates to the metal and a new C-C bond is formed. Carbon monoxide inserts again into the Pd-C bond and forms another C-C bond, which increases the length of the polyketone, see Figure 3.

Fig. 3

Scheme illustrating the synthesis of polyketones catalysed by a palladium complex. Carbon monoxide inserts into the palladium-methyl bond

Palladium in Chiral Catalysis

Chiral catalysis is a rapidly advancing field and the advent of chiral C-C bond formation is at the leading edge of this research. J. M. Brown, of Oxford University, described the palladium catalysed Heck reaction. He showed that for this reaction the yields and enantiomeric excess of the products depended on the base, see Figure 4.

Fig. 4

Scheme of a palladium catalysed Heck reaction leading to two isomeric pairs, A and B, of euantiomers

Base

Product A

Product B

Yield, per cent

e.e., per cent

Yield, per cent

e.e., per cent

1,8 C10H6(NMe)2

46

96

17

24

Cy2NH

59

82

4

43

Cy is cyclohexyl

The conference was very informative on new developments and progress in the use of the platinum group metals for catalysis. It is proposed that the second Anglo-Dutch Symposium will be held on 25 and 26 September 1997 in the conference hall of the Royal Academy of Arts and Science in Amsterdam. The organisers are Professors C. J. Elsevier, G. van Koten and K. Vrieze.